Protecting/mounting altimeters in small body rockets

[Joshua F Thomas]

The title says it all. You’ve got a bird small enough that it doesn’t have a payload bay, and you want to put an altimeter in for flight data - but you don’t want to have it destroyed by ejection charge gasses or bouncing around. What do?

I made my Flight Sketch Mini a small cotton “bag” out of a spare rag. It’s flown 4 or 5 times in my Der Red Max. I tied it up to the nosecone mount and the whole thing is shoved inside the nosecone.

Data not been great; I’m clearly getting bad numbers out of it. So what can I do here?

 

[Jay Rairigh]

...you do have vent holes in the nose cone?

 

[Ronz Rocketz]

Are you getting inconsistent data or does it not match with the sims?

 

[rharshberger]

As others have asked..
1) Do you have static ports in the airframe below the nosecone? If not it will pretty much detect launch and apogee, if the static ports are in the nose cone the data will be noisy.

2) get a BT60 coupler and short section of airframe and make a payload bay for the altimeter to ride in away from ejection events again put static ports in the airframe tube at least one airframe diameter behind the nose cone.

I have flown my two FS Mini's mulitple times just wrapped into the chute bundle of many LPR's (mostly my kids Estes Crayons).

 

[Joshua F Thomas]

The Der Red Max has two vent holes in the nose cone. The FS Mini has been mounted inside the nose-cone with my small cotton "bag" around it for ejection gas charges. It's been held by a small piece of kevlar to the nose-cone mount.

I launched yesterday on a C6-5 and a C5-3. The C6-5 looks to my eye like it's in the 400 ft range, based on other rockets I've launched to that altitude, but the FS Mini reported a max of 270 ft. The simulation says it should be somewhat over 400 ft. I also launched on a C5-3, and all I got was a brief spike to 40 ft for data.

Questions:

1) Why are vent holes (or static ports) in the nosecone not as reliable as in the airframe?
2) Are there concerns about holes in the airframe venting exhaust pressure, therefore reducing power of the ejection of the nosecone and the chute?
3) Is there a how-to on building a payload bay into an existing airfame?

I have flown my two FS Mini's mulitple times just wrapped into the chute bundle of many LPR's (mostly my kids Estes Crayons).

Where do you attach it to, the same mount as the chutes?

 

[heada]

Vent holes on the nosecone don't provide access to stable air to sample so your sampled pressure and thus altitude result will be inaccurate.

The holes needed to sample air pressure don't need to be very large on a small rocket. 3x 1/8" should work. That size hole should not impact deployment charges enough to give any issues and my method doesn't expose those holes until everything is in motion from the ejection charge.

The easiest way I've found to modify an existing rocket is to add a coupler and 2 bulkheads and mount the electronics in that coupler (add a section of BT and you can covert it into dual deploy). The coupler sits at the forward end of the rocket just below the nosecone. I glue a dowel rod to the inside of the BT from the forward centering ring up to the base of where I want the av-bay to be so that it doesn't slide down into the BT. Shock cord and parachute are attached to the aft end of the coupler, nosecone to the forward end. Prep the rocket like normal with motor, wadding and parachute then slide the av-bay in and rotate so that holes in the body line up with holes in the coupler and then add the nosecone. If you used the Estes tri-fold shock cord attachment method, you may need to remove it and re-attach lower in the rocket. This is one of the reasons I attach shock cords to the MMT using kevlar string when building all my rockets now.

 

[rharshberger]

Questions:

1) Why are vent holes (or static ports) in the nosecone not as reliable as in the airframe?

Assmentioned earlier nosecone static ports are sampling turbulent air, hence wonky readings, if its a deployment altimeter it will still detect and fire charges at apogee, but readings will still be wonky.
2) Are there concerns about holes in the airframe venting exhaust pressure, therefore reducing power of the ejection of the nosecone and the chute?
Static ports are usually small enough to have little and most generally zero effect on ejection pressures.

3) Is there a how-to on building a payload bay into an existing airfame?
Several methods, but a coupler with a bulkhead and eyebolt for parachute attachment and short section of airframe for nosecone to go into will make a functional payload bay.




Where do you attach it to, the same mount as the chutes?

I do attach the FS mini carrier to the nosecone or parachute attachment point.

 

[cerving]

Dino Chutes just came out with a "pouch" for the FS Mini, you can use this approach for other electronics that you are just using for data gathering (i.e. an Eggtimer Quantum with deployments turned off). I wouldn't do it for HPR, however for LPR/MPR it's "good enough". Drill a few small vent holes in the body tube for best results, although most rockets aren't all that airtight (which is why things like the JL A1 work without doing anything with the rocket).

 

[Joshua F Thomas]

The easiest way I've found to modify an existing rocket is to add a coupler and 2 bulkheads and mount the electronics in that coupler.

Uh, how? So on the bottom end you'll have to glue a bulkhead into the coupler and put an attachment point on it, like an eyebolt, so the shock cord can attach to it. If you're using two bulkheads, what are you doing with the other one? Gluing it in on the top, and connecting the nose cone to it with another link of shock cord?

How do you get the altimeter in and out of the coupler if you've glued bulkheads to each end? Or are you just leaving one end open and then putting stuff in between the bay and the nosecone?

 

[Joshua F Thomas]

Dino Chutes just came out with a "pouch" for the FS Mini, you can use this approach for other electronics that you are just using for data gathering (i.e. an Eggtimer Quantum with deployments turned off). I wouldn't do it for HPR, however for LPR/MPR it's "good enough". Drill a few small vent holes in the body tube for best results, although most rockets aren't all that airtight (which is why things like the JL A1 work without doing anything with the rocket).

I actually just saw that. Do you just let it swing free on the nosecone mount? Mount it to the side of the body tube some how?

 

[heada]

Uh, how? So on the bottom end you'll have to glue a bulkhead into the coupler and put an attachment point on it, like an eyebolt, so the shock cord can attach to it. If you're using two bulkheads, what are you doing with the other one? Gluing it in on the top, and connecting the nose cone to it with another link of shock cord?

How do you get the altimeter in and out of the coupler if you've glued bulkheads to each end? Or are you just leaving one end open and then putting stuff in between the bay and the nosecone?

I don't glue the forward bulkhead on, I use a section of 6-32 all thread down the side. Aft bulk head is glued in place with the all-thread attached via a nut. Forward bulkhead is kept closed via a washer and nut on that all-thread. Cords are attached on either end via screw-eyes. Its basically a mini av-bay like you'd use for dual deploy.

Check out Apogee for some commercial solutions. I use their BT60 av-bay with an Eggtimer Quark and a 400mAh 2S lipo for dual deploy on an Estes Star Orbiter (29mm MMT in a 38mm tube)
https://www.apogeerockets.com/Electronics_Payloads/Electronic_Bays

 

[Joshua F Thomas]

I don't glue the forward bulkhead on, I use a section of 6-32 all thread down the side. Aft bulk head is glued in place with the all-thread attached via a nut. Forward bulkhead is kept closed via a washer and nut on that all-thread. Cords are attached on either end via screw-eyes. Its basically a mini av-bay like you'd use for dual deploy.

One thread or two? I'm just thinking it feels not very secure on just one. Do you also have a sled in the middle?

 

[heada]

I use 1 threaded rod and a 1/16" ply sled just to mount the electronics to. This isn't a HPR solution, it has to withstand a few oz of weight (coupler, 1/16" ply bulkheads, screw eyes and nosecone). The majority of the force is on the aft bulkhead which is glued to the coupler.

 

[Joshua F Thomas]

The Der Red Max already has such a short body that if I do this, I'll have to build it into the nose cone; and apparently that isn't a good idea due to the pressure issue you mentioned. The body tube barely has enough room for the chute and chute protector as it currently stands. I'll probably just try the FS Mini inside a bag hanging off the nosecone, inside the chute.

 

[cerving]

I actually just saw that. Do you just let it swing free on the nosecone mount? Mount it to the side of the body tube some how?

If it's a baro unit, then it won't really matter much if it's a bit loose. It IS better to mount it in a "real" AV bay with a sled, of course, but for a Q&D data acquisition using a pouch or wrapping it up in a blanket works fine.

 

[BEC]

Come on, guys, we’re talking about a device that weighs a tenth of an ounce. This business about 6-32 threaded rod and such is overklll. Also...as the OP noted, this is in a Der Red Max...a BT-60-based model with a pretty short body.

I have many, many flights on FlightSketch Minis (339 flights, if I counted right, posted on the web flight log) by now. A few of those flights carried two Minis.

In models with no payload section I put the Mini in a small fleece pouch (these my wife made for me), put a loop of Kevlar through the hole in the corner of the board as a lanyard to tie it to the eyelet on the nose cone (generally), and stuff it in above the parachute. This works even on models as small as an Alpha.

The body of the model needs to be vented, preferably some distance below the nose cone/body tube joint, to give it relatively good access to ambient air pressure. In a DRM I’d put three 3/32 inch holes evenly spaced at least two inches down from the top. It doesn’t matter that the altimeter is above the holes or that when you pack the model the ‘chute is in front of them. It won’t seal the holes too much.

Now to the specific case: Yes, 269 feet on a C6-5 is kind of low for a DRM. But - was the flight straight or did it weathercock (or was it launched at an angle)? How heavy is it? Square edged fins or rounded? The actual flight data look reasonable (https://flightsketch.com/flights/1217/), with deployment a bit over the top and a bit of a jolt (or a pressure spike) at ejection. Also, it looks as if the ‘chute took its time opening all the way. It also rocked or spun quite a bit on the way down, it looks like.

But if your static ports are in the nose cone and therefore getting some dynamic pressure increase, the data will be off.

BTW there’s nothing wrong with putting the FS Mini in the nose cone, as long as the static ports are down on the body a ways.

You apparently didn’t post the C5-3 flight to the log. Sometimes a solid jolt (usually at ejection) will cause the FS Mini to loose power for a fraction of a second, and it will reset itself. If your max altitude on the app is reading zero after a flight, this is often the case. Here’s a recent flight of mine where I’m pretty sure that happened: https://flightsketch.com/flights/1193/.

This is another model where the FS Mini is flying in a little fleece pouch my wife made, and the static ports are a ways down the body.

I don’t think I’ve ever experienced this data truncation flying the Mini in a dedicated payload section. I suspect the banging around of nose cone, body, altimeter-in-pouch, and such at ejection is the culprit as those sensitive little baro sensors are also sensitive to getting whacked.

I’m not surprised Dinochutes has a pouch now. They had one for the PerfectFlite Firefly and if it will fit the Firefly it will fit the FS Mini.

I fly PerfectFlite FireFlies and occasionally Altus Metrum MicroPeaks in fleece pouches the same way as well.

 

[rharshberger]

My prototype FS Mini carrier, fits in a BT20...there is also a adapter for BT50 and larger in the works.
Printed in transparent PETG, its main purpose is to prevent the unit bouncing on the ground. Also thinking about a version for NAR Payload Competition

 

[BEC]

Cool, Rich. So 9.3g with the Mini in it. That means the little 3D printed container/lanyard weighs a little over twice as much as the Mini itself. Not sure I'd want that in a competition model, but for sport flying, that would be OK.

 

[o1d_dude]

My prototype FS Mini carrier, fits in a BT20...there is also a adapter for BT50 and larger in the works.
Printed in transparent PETG, its main purpose is to prevent the unit bouncing on the ground. Also thinking about a version for NAR Payload Competition

View attachment 429677 View attachment 429678 View attachment 429679 View attachment 429680 View attachment 429681

Very slick, Rich.

I just ordered a small pouch from Dino Chutes specifically for the FS mini. It’s probably heavier than your printed set up due to Velcro and such.

 

[Joshua F Thomas]

Now to the specific case: Yes, 269 feet on a C6-5 is kind of low for a DRM. But - was the flight straight or did it weathercock (or was it launched at an angle)? How heavy is it? Square edged fins or rounded? The actual flight data look reasonable (https://flightsketch.com/flights/1217/), with deployment a bit over the top and a bit of a jolt (or a pressure spike) at ejection. Also, it looks as if the ‘chute took its time opening all the way. It also rocked or spun quite a bit on the way down, it looks like.

But if your static ports are in the nose cone and therefore getting some dynamic pressure increase, the data will be off.

The flight you're linking was actually from a few days ago and was different - it *did* weathercock into the wind, and the wind was fairly strong. The 269 feet seemed to match my own experience for that flight. You also have a good eye for the chute; I was trying to reef lines and it didn't work as expected, came down faster than I liked.

You apparently didn’t post the C5-3 flight to the log.

Both of my two most recent flights had data that was strange enough I just didn't both uploading them.

Sometimes a solid jolt (usually at ejection) will cause the FS Mini to loose power for a fraction of a second, and it will reset itself. If your max altitude on the app is reading zero after a flight, this is often the case. Here’s a recent flight of mine where I’m pretty sure that happened: https://flightsketch.com/flights/1193/.

Ah, this is helpful info, thank you.


I've gone ahead and added three vent ports below the nose cone of the Der Red Max, and I'm waiting on my Dinochutes pouch, so I'll try again and I'll see how it goes. Thanks for the help!

 

[BEC]

The flight you're linking was actually from a few days ago and was different - it *did* weathercock into the wind, and the wind was fairly strong. The 269 feet seemed to match my own experience for that flight. You also have a good eye for the chute; I was trying to reef lines and it didn't work as expected, came down faster than I liked.

Lots of practice. As I noted, I've over 300 flights with FS Minis since I got a beta unit a year ago last March. The graph clearly shows a pretty fast descent and then there is a sudden change of slope and the descent rate decreased. It looks kind of like a graph from a Jolly Logic Chute Release flight.

Both of my two most recent flights had data that was strange enough I just didn't both uploading them.

Too bad....I'd liked to have seen the questionable C6-5 flight you originally asked about. Also it's interesting that it was to essentially the same reported altitude.

I've gone ahead and added three vent ports below the nose cone of the Der Red Max, and I'm waiting on my Dinochutes pouch, so I'll try again and I'll see how it goes. Thanks for the help!

You're welcome. It will be interesting to see how moving the static ports changes things. You will be filling/covering with tape/blocking the ones in the nose cone, of course, right?

 

[Joshua F Thomas]

You will be filling/covering with tape/blocking the ones in the nose cone, of course, right?

Is that necessary? Is the turbulent air flow from the nose cone holes enough to mess up the readings from the body holes?

I haven't gotten to fluid dynamics in my undergrad yet....

 

[BEC]

Yes....otherwise you’re still getting the dynamic pressure effects of airflow pushing into the body, raising the pressure in there, plus whatever turbulence is going on.

 

[Joshua F Thomas]

Hm, perhaps I can plug it with some thick CA.

 

[BEC]

Since I don’t actually know what your model looks like....I’m thinking in terms of my favorite clear tape - Scotch Multi-Task - over the holes in the nose cone.